To tickle means to lightly touch someone in a way that causes an involuntary, squirmy, laughing reaction. When someone tickles you under your arms or on the soles of your feet, your body responds with sudden laughter and the urge to twist away, even if you're trying to stay still. Scientists still debate exactly why we react this way, but the sensation seems to be our nervous system's response to light, unpredictable touches in sensitive areas.

The word can also mean to please or amuse someone in a delightful way. A clever joke might tickle your sense of humor, or a surprising compliment might tickle you pink (an expression meaning to make someone extremely happy). When something tickles your fancy, it catches your interest in a particularly pleasing way.

You might notice that you can't tickle yourself. The element of surprise matters: your brain needs to be caught off guard by someone else's touch for the tickling sensation to work. This is why even people who dislike being tickled will sometimes laugh uncontrollably when it happens.